FIG. 1 of the accompanying drawings is a schematic representation of the essential components and power flows in a typical two-regime CVT, in which change between regimes is made in a synchronous manner as well understood in the art. The output of engine E is connected both to the variator V, that is to say the ratio-varying component of the CVT, and also to a gearing unit G. The outputs of variator V and unit G constitute separate inputs to a gearing unit M, usually referred to as the "mixer" gear unit and of epicyclic type. Output O of unit M constitutes the output of the CVT. Controlled engaging devices G' and M' are associated with unit G and M respectively: devices G' and M' will be referred to as a brake and a clutch respectively, because that is what they are in several known transmissions.
The first of the two regimes in which the CVT is capable of operating is typically known as "low" regime. During this regime, brake G' is engaged and clutch M' open, so that the outputs of unit G and variator V drive two of the components of mixer M and the output O, taken from a third component, 1 s the resultant of the two inputs. Typically, with the CVT in low regime, a full sweep through the ratio range of variator V, beginning with the variator set to deliver high ratio, causes the speed of rotation of output O to progress from maximum reverse value through a state where O is at rest but the other components of units G and M are in motion and the CVT is said to be in "geared neutral", and then to rise in the forwards sense, reaching a relatively low value of forward speed when the variator V reaches the opposite--i.e. low--end of the ratio range to that at which it began. If the ratios of variator V and units G and M are appropriately chosen, and brake G' and clutch M' are respectively disengaged and engaged at this moment, the second or "high" regime may take over from low regime without any instantaneous change in the rotary speed of output O. Such a regime change is known in the art as a "synchronous change". Thereafter, if the ratio delivered by variator V is progressed back to the original end of its range, the forward velocity of output O increases steadily. At the moment of synchronous change, and throughout high regime, clutch M' "locks-up" mixer unit M, that is to say locks the three components of the epicyclic or like unit so that they rotate in unison. In high ratio the rotary speeds of output O and of the output of variator V are therefore the same, in both magnitude and direction, and unit G is inactive. FIG. 1 also indicates the power flows that take place within the CVT during forward motion in low regime. Assuming reasonable efficiency, power P.sub.o at output O will be approximately equal to the engine power P.sub.e, but power will recirculate in the "loop" of units G, M and V much as indicated, the power in two of the connected limbs being greater than engine power by an increment .delta., and the power in the remaining two limbs being equal to that increment. As is well understood in the art, if its to be possible to achieve a condition in which the three components of mixer unit M are locked up and so permit a synchronous change between low and high regimes, two conditions should be fulfilled. Firstly, the magnitude of the input from unit G to mixer unit M must be equal, both in magnitude and direction, to the input that it receives from variator V when that variator is at the low ratio setting at which synchronous change is made. Second, somewhere within the power recirculation loop, a reversal of rotation must be achieved, because there is a necessary reversal of direction between the input and output of a variator of the toroidal-race rolling-traction type, and the system must therefore include a further reversal of direction in order for the senses of rotation of the two inputs to the mixer unit M to be the same, as is necessary.
Hitherto it has been customary, in the art, to achieve both the reversal and the speed reduction within unit G itself. This happens, for instance, within the speed-reducing unit 107 shown in FIG. 4 of Patent Specifications GB-A-2023753, GB-A-2100372 and U.S. Pat. No. 4,297,918. The inevitable consequence of this combination of requirements has been to reduce somewhat the potential efficiency of unit G, which like unit M may be of epicyclic type, like item 107 in the two patent publications just recited.